Today, there is a rapid increase of data traffic in mobile networks, due e.g. to increased use of mobile broadband and smartphone applications. This increase in data traffic results in heavy demands for more capacity in the mobile networks. To meet these demands, there is a need for a large densification of radio base stations (RBSs), and heterogeneous networks are envisioned, comprising cells of different sizes, such as e.g. so-called macro cells, pico cells and femto cells. The RBSs serving these cells need to be connected to a core network. The link between an RBS and a core network is often referred to as a backhaul link.
The traditional alternatives for a backhaul link for a macro cell are microwave (MW), fiber and leased line copper. However, for small cell indoor scenarios there is limited availability of fiber and not always possible to use MW links. Instead, it may be required and/or suitable to reuse existing wire lines, such as telephone graded copper cables or CAT3 & CAT5. In such a scenario, small cells like pico cells or femto cells will use ADSL2/ADSL2plus, VDSL2, or the coming new standard G.fast for mobile backhaul over such reused wire lines. As it is assumed that 80% of all radio traffic will be generated indoor, this is also where an increased densification of small cells, e.g. pico cells, will take place. FIG. 1 illustrates a scenario in a building 106, where Radio Base Stations (RBSs) 101a-c and 103a-c, serving small cells, are backhauled with ADSL2/ADSL2plus, VDSL2 or G.fast over reused wire lines 102a-b. RBSs 101b-c and 103b-c are not connected directly to wire lines 102a-b, but are connected via Gigabit Ethernet (GbE) connections. This type of connection of RBSs in series is referred to as that the RBSs 101b-c and 103b-c are “cascaded”. The backhaul of the cascaded RBSs depend on the connection over wire line 102a and 102b, respectively. FIG. 1 further shows RBSs 104 and 105, which are connected to fiber backhaul links 106a-b. The different wire lines and cables providing backhaul links to the RBSs are connected to an aggregation node 107 for further communication with core network etc.
In the radio networks of today there are often redundant backhaul links between a Macro cell and a core network, which minimizes the risk of total service outage due to backhaul failure. However, in a heterogeneous network with an increased amount of small cells located indoors, which in many scenarios will reuse existing copper cables, it will not be possible to provide redundant backhaul links, and in the event of error on the backhaul the whole link will be affected. Furthermore, for heterogeneous deployment with small cells that are backhauled with VDSL2 or G.fast, the small cells will with high probability be placed in locations where the backhaul may be exposed to disturbances that could impact the backhaul performance.
When a backhaul link is exposed to interference, the result may be that the Signal to Noise Ratio on the backhaul link will decrease such that the configured bit rate cannot be maintained. There are three possible outcomes from this situation e.g. in a VDSL2 system:    1. Perform a Seamless Rate Adaptation, SRA, which enables to reduce the bit rate without a retrain. This process may take several seconds to be finished and requires negotiation between VDSL2 DSLAM and CPE.    2. Save Our Showtime, SOS, which is a mechanism where a reduction of rate is achieved through a flat reduction of the bit load over groups of tones. The SOS results, in general, in a bit rate which is far lower than what is achieved with SRA or Retrain. SOS may be used e.g. if the disturbance is so bad that an SRA will not be possible.    3. Perform a Retraining, where a lower rate corresponding to the reduced SNR on the line will be achieved. This would also disrupt the radio channel for the duration of the re-initialization of the backhaul.            a. Vectored VDSL2 retrain of one line could take up to 20-90 seconds, if several lines need to be retrained, the startup time will be even longer.        
The capacity of the backhaul link will thus either be zero (for a certain time period at retrain) or reduced to a lower number. This may cause serious problems to the users located in a cell, when these users are engaged in services which require that data is transmitted over the backhaul link. That is, the users in such a cell may suddenly have no, or at least a severely reduced, possibility to communicate with other entities via the backhaul link.